The present invention represents improvements in the apparatus and method disclosed in U.S. Pat. No. 4,348,057 of Parenti, et al. The machine disclosed in that patent includes a large-capacity main blower for drawing air from a location adjacent the face of the coal being mined. The main blower draws the air through a grid which deflects large pieces of coal into a refuse bin. After passing through the grid, the air passes through a main stage of centrifugal filters which separates dust and particles into a stream of dirty air and discharges the clean air back into the mine. The dirty air is routed through a secondary stage of centrifugal filters which is energized by a second blower, sometimes referred to as a pressure unit. In an effort to further clean the air before it is discharged back into the mine and particularly to remove very small particles and respirable dust, the output of the pressure unit is routed through a cloth filter stage and thence into the atmosphere.
The cloth filter stage is considered to be a significant part of the machine because, whereas the centrifugal filters are capable of handling larger volumes of air, they are not capable as a practical matter of filtering out very tiny particles having a size of 10 microns or less, and commonly referred to as "respirable" dust. It is just such respirable dust which may be most harmful to miners. Thus, as a practical matter, it is important to include some type of cloth or other screening filter capable of removing respirable dust from the air during processing and before the clean air is discharged back into the mine.
However, a problem arises in the operation of the machine described above in that after the machine is used for some period of time (e.g., a few hours), the cloth filter begin to clog up as they filter out dust and smaller particles. The machine is equipped with a shaking device for cleaning the cloth filters, but as a practical matter the cloth filters may become congested more quickly than a normal maintenance routine or shaking of the filters would clear them. In such a case, that is, should for any reason, the cloth filters become clogged, the operating pressure for the secondary stage of centrifugal filter may be affected in a way which reduces its effectiveness for separating out dust or particles, thus further exacerbating the problem of clogging the cloth filter.